1. Field of the Invention
The present invention demonstrates the method of fabricating porous silicon metal-semiconductor-metal planar photodetector.
2. Description of the Prior Art
Photodetector is a device that can convert light signal into electrical signal. It is widely used in various areas like communication, computer, control, and medical applications. Since silicon cannot emit light efficiently, optoelectronics is developed in the area of compound semiconductor. So far high illumination light emitting diode has been developed from porous silicon, therefore if there is a corresponding planar photodetector, then silicon integrated optoelectronic circuit can be developed. Metal-semiconductor-metal planar photodetector (M-S-M) is of great potential to be applied in optoelectronics integrated circuit since (1) it""s structure is planar, (2) no minority carrier storage and the distance between electrodes is short so that the response is fast, (3) simple to fabricate, low cost, (4) high electro-optics conversion efficiency, and also high stability photodetector is demanded by the market widely.
There are four advantages to apply porous silicon in photodetector: (1) since it is direct bandgap, so the elector-optics conversion efficiency is high, (2) porous surface can increase the absorption rate of light (3) it has high gain due to avalanche effect, (4) simple to fabricate, low cost, so porous silicon is a good photodetecting material. The present invention fabricates a high gain and low leakage current porous silicon metal-semiconductor-metal planar photodetector.
Silicon is the most matured semiconductor material developed so far. well The integrated circuit technology of silicon is well developed. Optoelectronic integrated circuit is the future integrated circuit which has faster speed, since it transmits signal with the speed of light. Photodetector is the necessary device of optoelectronic integrated circuit in the respect of function and structure. Metal-semiconductor-metal photodetector is planar photodetector, specially suitable to be used in optoelectronic integrated circuit. Furthermore, since it has no minority carrier storage and the distance between electrodes is short so the response is fast.
Porous silicon differs from bulk silicon, according to the report of L. T. Canham in Appl. Phys. Lett., Vol. 57, pp.1046 (1990), after the silicon surface is etched by the current in electrolytic solution and becomes porous, the energy band is folded and its indirect energy gap becomes direct bandgap, so that the electro-optics conversion efficiency is increased, therefore the quantum coefficient of photon is increased. At the same time since the surface of porous silicon is porous which can eliminate the secondary reflection of light, so the absorption rate of light is increased, we can obtain better photocurrent and photosensitivity without antireflection coating.
Additionally, the wire size of the porous silicon is very narrow, and it is in complete depletion, so the avalanche effect in the local region will occur due to bias and amplify the optical signal, and thereby the photodetector will have high gain.
From the above, there are five advantages to apply porous silicon in photodetector: (1) since it is direct bandgap, so the electro-optics conversion efficiency is high, (2) porous surface can increase the absorption rate of light (3) it has high gain due to avalanche effect, (4) directly formed on silicon wafer so that it is totally matched with silicon integrated circuit. (5) its fabrication process and structure are simple and the cost is low, so porous silicon is a good photodetector material.
But as-grown porous silicon photodetector has several disadvantages: (1) optical and electrical characteristics are not stable, (2) photocurrent and photosensitivity is not high enough, (3) dark current is not low enough, which limits the application of photodetector.
The following depicts the problems hereby resolved by the present invention.
So far the development of metal-semiconductor-metal porous silicon photodetector still has the following problems: (1) photocurrent and photosensitivity is not high enough, (2) dark current is not low enough, which limits the application of planar metal-semiconductor-metal photodetector in optoelectronic integrated circuit.
The present invention applies the method of rapid thermal oxidation (RTO) and rapid thermal annealing (RTA) to fabricate high gain and low leakage current porous silicon metal-semiconductor-metal planar photodetector. The photocurrent and photosensitivity of the present invention is raised high, and the dark current of the present invention is reduced, which makes the application of planar metal-semiconductor-metal photodetector in optoelectronic integrated circuit to be more widespread.